/Engine/Source/Runtime/Engine/Public/AnimationRuntime.h
This blends the rotations in mesh space, so is performance intensive as it requires a conversion from joint rotations into mesh space, then the blend, then a conversion back.
static void BlendPosesTogetherPerBoneInMeshSpace ( [TArrayView](API\Runtime\Core\Containers\TArrayView)< [FCompactPose](API\Runtime\Engine\FCompactPose) > SourcePoses, [TArrayView](API\Runtime\Core\Containers\TArrayView)< const [FBlendedCurve](API\Runtime\Engine\Animation\FBlendedCurve) > SourceCurves, const [UBlendSpace](API\Runtime\Engine\Animation\UBlendSpace) * BlendSpace, [TArrayView](API\Runtime\Core\Containers\TArrayView)< const [FBlendSampleData](API\Runtime\Engine\Animation\FBlendSampleData) > BlendSampleDataCache, [FCompactPose](API\Runtime\Engine\FCompactPose) & OutResultPose, [FBlendedCurve](API\Runtime\Engine\Animation\FBlendedCurve) & OutResultCurve )